1. Field of the Invention
The present invention relates to a process for producing a negative-crowned roller.
2. Description of the Prior Art
For example in an electrophotographic apparatus, for the purpose of fixing a toner image onto a support material such as plain paper there is commonly employed a roller fixing apparatus in which the support material having an unfixed toner image is supplied to and advanced between at least a pair of rollers maintained in mutual pressure contact. Such roller fixing apparatus utilizes for example the thermal fixing process wherein at least one of said paired rollers is heated for melt-fixing said toner onto the support material or the pressure fixing process wherein said paired rollers are mutually pressed with a linear pressure of 10 kg/cm or higher by which the toner image is fixed onto said support material. In these processes there should be provided a suitable measure for preventing wrinkle formation on the support material during the advancement thereof between the paired rollers, and for this purpose it is already known to shape either or both of said paired rollers in the form of roller 1 shown in FIG. 1, said form having a negative crowning in which the diameter, contrary to the so-called crowning, progressively decreases from the both ends towards the center of the roller. Such roller, in cooperation with an ordinary straight roller or a similar negative-crowned roller, exerts on the support material to be advanced between said rollers an extending force in the transversal direction of said support material (i.e. a direction parallel to the axial direction of said rollers or perpendicular to the advancing direction of said support material) from the center toward the lateral edges of the support material, thereby preventing the wrinkle formation thereon. Said negative-crowned roller is preferably elastically deformable at least at the surface portion thereof so as to be in intimate contact with the other of the paired rollers when maintained in pressure contact therewith. In the above-explained fixing apparatus, therefore, there is preferred the use of a rubber roller composed of a metal core covered with a silicone rubber layer of which peripheral surface is shaped into the negatively crowned form. The silicone rubber is also useful for preventing the offsetting of the toner.
In order to prepare such a negative-crowned roller as explained above, it has been necessary to move the support of a grinding roller in a direction parallel to the axis of the roller to be ground and simultaneously to move the grinding roller itself in the radial direction of the roller to be ground along a curved trajectory. Naturally it is alternatively possible to move the roller to be ground in the corresponding directions to achieve such mutual movements, but in any case a two-dimensional movement has to be made either by the grinding roller or by the roller to be ground, thus requiring a complicated working process and device, eventually resulting in a deteriorated precision and an elevated manufacturing cost.
Though such negative-crowned roller can also be prepared inexpensively by solidifying a liquid material in a female mold for such roller, it is difficult to obtain a smooth roller surface with an enough precision. The negative-crowned roller to be employed in the toner image fixing apparatus, however, is required to have satisfactorily high dimensional precision and surface smoothness, and also to be reasonably inexpensive.